Objectives: This is an in vitro study comparing the flexural strength of two dental porcelain after thermocycling, repetitive multiple firings and peak firing temperature (PFT) modifications.Materials and Methods: Two types of dental porcelains were used in this study: Cercon® Cream Love and Cercon® Cream Kiss (DeguDent, Hanau-Wolfgang/Germany). A total of 50 specimens per porcelain were divided into five groups with ten specimens per group. The first group was fired following the manufacturer's recommendations (control). The second group was fired 10 times repetitively, and the third group was fired and then exposed to 3000 episodes of thermocycling. The fourth group was fired with the PFT modified by + 10°C. The last group was fired with the PFT modified by −10°C. All specimens were tested for flexural strength with the three-point bending test using a universal testing machine (Instron) with a crosshead speed of 0.5 mm/min.Results: The flexural strength, mean (standard deviation [SD]) MPa, of Cercon® Ceram Kiss were: Control, 59.6 (7.82); 10 firings, 82.37 (26.32); thermocycling, 68.93 (9.55); PFT (+10°C), 75.28 (5.63); and PFT (−10°C), 75.94 (2.85). The results for the flexural strength, mean (SD), of Cercon® Ceram Love were: Control, 59.33 (9.9); 10 firings, 106.07 (28.43); thermocycling, 69.87 (11.17); PFT (+10°C), 65.7 (10.46); and PFT (−10°C), 51.1 (16.56).Conclusion: There was no difference in the flexural strength between the two veneering porcelains. In addition, repetitive firings significantly improved the flexural strength of both porcelains. Thermocycling significantly increased the flexural strength of both porcelains. There were no significant differences in the flexural strength when changing the PFT for Cercon® Ceram Love. Increasing and decreasing the PFT significantly increased the flexural strength of Cercon® Ceram Kiss.

Patients' cosmetic demands have increased the production of new esthetic materials. Cosmetic materials introduced in the last decade have started to improve esthetic results with downgraded functional and physical properties. Zirconia-based restorations have shown to be the closest integration between esthetic and functional harmony. Its high flexural strength and superb mechanical properties have tempted restorative dentists and prosthodontists to use zirconia-based restorations in the anterior area, where more esthetic characteristics are required, as well as the posterior area, where a higher load on the restorations is expected. [1] Documented clinical failures for zirconia-based restorations are related mostly to the limitations within the veneering porcelain. [2],[3],[4],[5] Therefore, further evaluation should be conducted of the physical properties of veneering porcelain and the effects of different variables during the production and after insertion of the final prosthesis.

The expected esthetic and functional results for the final restoration require several steps of layering buildup and repeated firings for veneering porcelains. Multiple firings have been shown to alter the physical properties of porcelain, such as the flexural strength and the bond strength of the core material. [6],[7],[8] In addition, production laboratories have a tendency not to follow proper calibration and maintenance programs for porcelain furnaces. [9] Technicians also tend to modify the recommended programming to obtain better results, [9] which may subsequently produce higher/lower peak firing temperatures (PFTs). For these reasons, porcelain may be exposed to uncontrolled heating/cooling environments with changing variables.

Following insertion of the final restorations in the mouth, porcelain will age within the oral cavity, undergoing continuous and intermittent hot/cold environments that might further affect its physical and mechanical properties. Porcelain will also be more prone to crack propagation with exposure to moist environments. [10]

This study will assess the effects of multiple firings, which are essential for porcelain buildup; the effects of PFT changes, which may be associated with technicians' modifications and/or improper calibration of porcelain furnaces; and the effects of thermocycling on the flexural strength of two zirconia veneering porcelains.

Materials and methods

Two types of zirconia veneering ceramics with a dentin body color, (Cercon Ceram Kiss and Cercon Ceram Love) were used in this study [Table 1].

Porcelain powder and liquid were mixed in a vacuum mixing machine. The mixture was then condensed into a silicon mold with dimensions of 35 mm × 4 mm × 5 mm. After vibration-condensation procedures, specimens of each ceramic were processed and prepared as follows:

Control, which was fired according to the manufacturer's recommendations [Table 2].

Fired according to the manufacturer's recommendations 10 times repeatedly.

Fired according to the manufacturer's recommendations and then thermocycled in distilled water at 5°C and 55°C for 3000 cycles with a dwell time of 15 s.

Fired with a 10°C higher PFT.

Fired with a 10°C lower PFT. A porcelain vacuum furnace (Cergo Press, DeguDent, Germany) was used for firing the ceramics.

Specimens were polished and trimmed to final dimensions of 3 mm in width, 4 mm thickness and 30 mm in length using diamond discs. The specimens were polished with a Meta Serv 250 Polisher-Grinder. A breakdown of the specimens and how they were divided for the following procedures is shown in [Figure 1].

Statistical analysis was performed using SPSS version 17.0, SPSS, Inc., Chicago IL, USA. For descriptive statistics, the mean, standard deviation, and graphs were used to present basic results. For tests of significance, ANOVA was used to find any significance between experimental groups. The level of significance was a P < 0.05.

Results

A summary of the results is shown in [Table 3], [Figure 2] and [Figure 3]. The highest flexural strength was reported to be 106.07 MPa, which was attained after firing Cercon Ceram Love 10 times; whereas the lowest values were 59.6 and 59.3 MPa for Cercon Ceram Kiss and Cercon Ceram Love, respectively. There was no significant difference in flexural strength between Cercon Ceram Kiss and Cercon Ceram Love. Significant increases in flexural strength were observed with repetitive firing (P < 0.05) and with thermocycling (P < 0.05). There were significant differences with both decreased and increased PFT for Cercon Ceram Kiss. On the contrary, there was no significant difference in flexural strength with increased or decreased PFT for Cercon Ceram Love.

This study was designed to assess the effects of temperature and aging treatments on veneering porcelains. Heat changes can have adverse effects on the mechanical properties of porcelains. [10] The three-point bending test has been previously used for the evaluation and comparison of flexural strength in dental porcelains. [11],[12],[13]

The strength values in this study for Cercon Love and Cercon Kiss were within acceptable ranges for flexural strength compared to those reported in the literature for veneering porcelains. [14],[15] There were no differences in the strength results between Cercon Love and Cercon Kiss. Both materials are manufactured by the same company (DeguDent, Germany), and they would be expected to have similar features of composition and mechanical properties. Cercon Love provides the upgraded feature of more diversity in shade matching and color.

Several repetitive firings can change the surface texture of the final restorations. [16] However, they are essential for fabricating an optimal and more adaptive prosthesis. In that respect, this study showed similar behavior by both materials in reaction to several firings. Repetitive firings significantly improved the flexural strength. This finding coincides with those in the literature. [8] The reason for this effect is most likely enhancement of the fusion between porcelain crystals after several firings. This process of fusion will increase the density of the porcelain, reduce porosity and subsequently enhance the mechanical properties.

Thermocycling is an important procedure that mimics the thermal cycling fatigue that occurs in the mouth. Several protocols have been used in the literature with different parameters for the number of cycles and the temperatures used, as well as the dwell time, but no particular protocol has been deemed to be superior. In this study, thermocycling was performed with 3000 cycles at 5°C and 55°C with a dwell time of 15 s to resemble approximately 1 year of service in the oral cavity. The flexural strength of the porcelain was significantly increased in this study after thermocycling. Some studies have shown decreases in the flexural strength of dental ceramics after thermocycling. [17],[18] Although the improvement in strength was found to be statistically significant, it is limited, and would most likely be of no clinical relevance.

The firing of porcelain for prosthetic restorations is a delicate and important procedure. Technicians tend not to calibrate their furnaces, and they also prefer to modify the recommended protocols to overcome noticeable defects within the veneering layer. [9] When the necessity for several porcelain firings is added, it is important not to overlook the possible consequences for the properties and the structure of the fired material. In this study, the flexural strength of Cercon Ceram Kiss increased with both higher and lower PFTs. The increase in strength with the higher PFT is mainly due to the above-mentioned improvement in density and reduction of porosity. However, further examination of the crystallization and microstructure of porcelain is needed to evaluate the improvement in strength with lower PFT. On the other hand, there was a steady increase in the strength of Cercon Ceram Love, but no significant differences were detected.

Further evaluation in a scenario that more closely resembles clinical use and/or microstructural studies are needed to reach a final conclusion on the behavior of porcelain undergoing various temperature changes.

Conclusion

Within the limitations of this study, it has been shown that multiple firings can be effective in improving the densification and the hardness of porcelains for zirconia restorations. The flexural strength was significantly increased with thermocycling. Modifying the PFT by 10°C had significant effects on Cercon Ceram Kiss. Thus, the precise control of condensation and ﬁring techniques is necessary to diminish porcelain shrinkage and stress production in the core and veneer interface buildup. However, all-ceramic restorations may need multiple heat treatments, which are required for the condensation process of veneering porcelain for modifications.

Acknowledgment

The author would like to acknowledge the services provided by Dr. Hamad Alrumaih and Mr. Lindsey Mateo for their assistance with specimens preparation and Mr. Soban Qadir for help with statistics.

Yilmaz K, Ozkan P. Profilometer evaluation of the effect of various polishing methods on the surface roughness in dental ceramics of different structures subjected to repeated firings. Quintessence Int 2010;41:e125-31.